1. Field of the Invention
This invention relates to a focus detecting device used in an optical apparatus in which the observation, measurement, and examination of an object are carried out through an optical system.
2. Description of Related Art
In an optical apparatus in which the observation, measurement, and examination of an object are carried out through an optical system, for example, in an optical microscope, an observer must perform focusing, in order to observe a sharp image of the object, by moving the stage of the microscope vertically to adjust a distance between an objective lens and the object. In this case, if the objective lens has a high magnification, a depth of focus is small, and thus a focusing position cannot be found when the stage is widely moved. Hence, the observer must move the stage little by little and needs much time to find the focusing position. On the other hand, if the objective lens has a low magnification, the depth of focus is larger, and thus, sometimes, it becomes difficult that the observer determines an optimum position of the stage where the object is brought to a focus.
In order to solve such a problem, a focus detecting device has recently become combined with the optical apparatus. Various systems are available in focus detecting devices. One of them is an active system focus detecting device in which light is radiated toward an object and reflected light from the object is detected by a photodetector to determine an in-focus or out-of-focus state in accordance with the reflected light.
The arrangement of the active system focus detecting device is shown in FIG. 1. In this figure, reference numeral 4 represents a light source; 5, a collimator lens; 7, a light-blocking plate; 8, a polarization beam splitter; 11, a quarter-wave plate; 12, dichroic mirror; 3, an objective lens; 13, an imaging lens; and 14, a photodetector. Reference symbol S represents a sample which is an object.
The light source 4 is a semiconductor laser, which emits laser light in an infrared wavelength region. This light is linearly polarized. The laser light is changed by the collimator lens 5 into a parallel beam, which is incident on the polarization beam splitter 8. In this case, by the light-blocking plate 7 interposed between the collimator lens 5 and the polarization beam splitter 8, a half of the light beam is blocked. The polarization beam splitter 8 has the characteristics of reflecting the linearly polarized light of p polarization and of transmitting the linearly polarized light of s polarization. Thus, when the semiconductor laser is previously placed so that the orientation of polarization of the laser light coincides with that of the p polarization, all the laser light incident on the polarization beam splitter 8 is reflected by the reflecting surface of the polarization beam splitter 8, and hence the intensity (amount) of light is not lost.
The laser light reflected by the reflecting surface of the polarization beam splitter 8 is incident on the quarter-wave plate 11. The quarter-wave plate 11 is placed so that the linearly polarized light incident thereon is changed to circularly polarized light, which emerges therefrom. The laser light emerging from the quarter-wave plate 11 is reflected by the dichroic mirror 12 and is incident on the objective lens 3. The objective lens 3 converges the laser light on the sample S.
The laser light reflected by the sample S passes again through the objective lens 3. At this time, the laser light does not follow the same optical path as in the case of incidence, but takes an optical path on the opposite side with respect to the optical axis. The laser light is reflected by the dichroic mirror 12 and enters the quarter-wave plate 11. Here, the laser light which is the circularly polarized light is changed to the linearly polarized light and emerges therefrom. However, since the orientation of the linearly polarized light becomes identical with that of the s polarization, all the laser light incident on the polarization beam splitter 8 passes through the polarization beam splitter 8 and enters the imaging lens 13. The imaging lens 13 condenses the laser light incident thereon. The photodetector 14 is located at a position where the laser light is condensed, and produces an electric signal in accordance with the intensity of the laser light. The photodetector 14 is such that two independent light-receiving sections A and B are arranged closely adjacent to each other. For example, a binary photodiode could be used.
In the arrangement shown in FIG. 1, the laser light collected on the sample S through the objective lens 3 is practically circular in shape and assumes a convergent point having an extremely small area (which is hereinafter referred to as spot light or a spot beam). The spot light has one spot. Such a construction is hereinafter termed a single-spot projection system.
In the single-spot projection system, how in-focus and out-of-focus states are decided (detected) is explained below with reference to FIG. 2A and FIGS. 3A-5B. FIG. 2A shows a state where the spot light is radiated on a convex surface of a sample of irregular shape and is brought to a focus on the convex surface. Specifically, a position where the size of the spot light collected by the objective lens 3 is minimized (which is hereinafter referred to as the focal position of the objective lens) coincides with the convex surface of the sample S. In this case, the spot light reflected by the convex surface, as shown in FIG. 4A, is converged at the center of the photodetector 14. For reference, the intensity distribution of a convergent beam (the spot light) is shown on the right side of the photodetector 14.
The photodetector 14 is constructed with the two light-receiving sections A and B which are identical in shape. A slight space (simply indicated by a solid line in the figure) is provided between the light-receiving sections A and B and coincides with the optical axis.
As seen from FIG. 4A, in the in-focus state, reflected light from the sample S is collected on the optical axis, and thus the spot light formed on the photodetector 14 has an intensity distribution of bilateral symmetry with respect to the optical axis. Specifically, since half of the spot light is formed on the light-receiving section A and the remaining half is formed on the light-receiving section B, the areas (intensities) of the spot light formed on the light-receiving sections A and B are equal. Hence, in the in-focus state, electric signals produced from the two light-receiving sections A and B are also equal.
In out-of-focus states, there are cases where the sample S is located at a distance away from the objective lens 3 with respect to the focal position and at a distance closer to the objective lens 3. Here, the former case is called a rear focus state and the latter is called a front focus state. In the rear focus state, as shown in FIG. 3A, the laser light reflected from the sample S is collected in front of the photodetector 14, and thus a light beam of a larger diameter than in FIG. 4A is formed on the photodetector 14. Furthermore, the light beam formed over the two light-receiving sections A and B has no spot of bilateral symmetry, and a larger part of the light beam is formed on one light-receiving section, namely the light-receiving section B. In the rear focus state, therefore, the electric signal generated in the light-receiving section A is smaller than in the light-receiving section B. Conversely, in the front focus state, as shown in FIG. 5A, a larger part of the light beam is formed on the light-receiving section A, and thus the electric signal generated in the light-receiving section A is larger than in the light-receiving section B.
As mentioned above, since the magnitude of the electric signal changes with the space between the objective lens 3 and the sample S, it is possible to decide whether the in-focus state or the out-of-focus state is brought about or whether the front focus state or the rear focus state is brought about in accordance with the value of a signal of the difference between the magnitudes of different signals (which is hereinafter referred as to a focus error signal). Hence, when such a focus detecting device is combined with an optical apparatus such as a microscope and the stage is moved vertically so that the focus error signal becomes zero, the sample can be brought to a focus automatically.
Another active system focus detecting device is set forth, for example, in each of U.S. Pat. Nos. 5,714,749 and 5,892,622. In these Patents, an arrangement is made such that a cylindrical lens or a toric lens is placed in an optical system. In such an arrangement, as shown in FIG. 6A, the laser light collected on the sample S by the objective lens 3 assumes an elongated slit shape. Also, such a system is called a slit projection system.
In the single-spot projection system shown in FIG. 1, when the spot light, as in FIG. 2B, is radiated at the edge (boundary) between concave and convex portions or at the edge of a step, the light is scattered at the edge. Consequently, a problem arises that the intensity of the light turning back to the photodetector is materially reduced and the accuracy of focus detection is degraded. Moreover, there is another problem that the shape of the spot light formed on the photodetector 14 is changed and, for example, the in-focus state is erroneously decided as the out-of-focus state because the focus error signal does not become zero, irrespective of the fact that the position of the sample S coincides with the focal position of the objective lens 3.
As shown in FIG. 2C, when a sample has an irregular shape of a plurality of different heights, only a portion of a particular height in which the spot light is radiated is brought to a focus. Thus, for example, if the highest surface is in focus, other concave and convex portions will be quite blurred, and it becomes impossible to observe a plurality of steps of different heights at the same time and to measure the widths of a plurality of steps at the same time.
In the slit projection system, as shown in FIG. 6B, even though slit light is radiated at the edge between concave and convex portions or at the edge of a step, the area of the light radiated on the planes of the concave and convex portions is larger than that of the light radiated at the edge. Consequently, light scattered by the edge has little effect on the radiation, and an error is rarely caused to the focus error signal. As in FIG. 6C, when a sample has an irregular shape of a plurality of different heights, light reflected by the portions of different heights is collected on the photodetector 14 at the same time, and hence a portion of a particular height is not brought to a focus, but the portion of an average height comes to a focus. Therefore, various concave and convex portions can be observed simultaneously.
However, the cylindrical lens and the toric lens which are used in the slit projection system are expensive by themselves. Since positional adjustment on assembly is difficult, a complicated adjusting mechanism is required. Moreover, in the irregular shape of a plurality of different heights, the portion of the average height is always brought to a focus, and it is impossible to bring the portion of the particular height to a focus with accuracy as in the single-spot projection system.
For the slit projection system, the method that a stop with an elongated aperture is provided in a light beam to form it into a slit shape is also available, but there is the problem that most of the light beam is blocked by the stop and thus a great loss of the intensity of light is caused.
It is, therefore, an object of the present invention to provide a focus detecting device which is not affected by the scattering of light at the edge of a step.
It is another object of the present invention to provide a focus detecting device in which in an irregular shape of a plurality of different heights, not only can a portion of an average height be brought to a focus, but a portion of a particular height can also be brought to a focus.
It is still another object of the present invention to provide a focus detecting device in which a loss of the amount of light is small, cost is low, and adjustment on assembly is easily made.
It is a further object of the present invention to provide a focus detecting device which gives a strong probability of focusing.
The focus detecting device of the present invention includes a multi-beam producing member for emitting a plurality of light beams; a light-blocking member for blocking a part of the plurality of light beams; a beam splitting member having a surface for reflecting or transmitting an incident light beam; a light-condensing optical system for condensing the incident light beam; and a photodetector having at least two light-receiving sections. The multi-beam producing member and the light-blocking member are placed on a first optical path, and the light-condensing optical system and the photodetector are placed on a second optical path. The beam splitting member is located at the intersection of the optical axis of the first optical path with the optical axis of the second optical path, and the photodetector is located at a position where the light beam is condensed by the light-condensing optical system.
The focus detecting device has a first driving mechanism for moving the diffraction optical element, which is moved along the first optical path.
The focus detecting device further includes an intensity attenuation member for reducing the intensity of an incident light beam and a second driving mechanism for moving the intensity attenuation member. When the diffraction optical element is inserted in the first optical path, the intensity attenuation member is removed from the first optical path, and when the diffraction optical element is removed from the first optical path, the intensity attenuation member is inserted in the first optical path.
The focus detecting device also has a beam adjusting mechanism for changing the number of a plurality of light beams produced by the multi-beam producing member or the spacing between them.
These and other objects as well as features and advantages of the present invention will become apparent from the following detailed description of preferred embodiments when taken in conjunction with the accompanying drawings.